At least three people are dead and 20 people injured after this act of incomprehensible horror, but the pain will ripple for years to come.

After the attack, a woman stood near the corner of Bourke and Williams streets and screamed at passersby, "Where is my baby? Where is my baby!" as she frantically searched for her child.

Minutes earlier, people had watched horrified as a maroon sedan bore down on pedestrians, including a woman pushing her child in a pram.

According to multiple reports, the force of the impact sent the child flying into the air, before they landed heavily on the concrete.

The pram was caught beneath the car and dragged metres down the street. Afterwards, it lay on its side, abandoned, near the ruined car.

Former police commissioner Christine Nixon, almost hit herself near the corner of Bourke and William Streets, came to the woman's aid.

"She kept asking me 'where's the baby?'," Ms Nixon told ABC radio. "And - I don't know."

Chief Commissioner Graham Ashton said the child was one of 20 people injured in the melee - five of them critically. At least one of those critically injured was an infant.

Royal Children's Hospital chief of surgery Mike O'Brien said a three month old baby was in surgery at the Royal Children's Hospital about 5pm on Friday afternoon, while a two year old fought for life in the intensive care unit.

It's understood that one child died at the scene. More may die.

The man responsible began his rampage of terror doing burnouts in a maroon sedan in the intersection in front of Flinders Street Station, just after 1.30pm.

At first, witnesses didn't know how to react. Was he drunk? Was he high?

He leaned out his car window, swearing at people, and giving them the middle finger.

Then he took one final lazy circle of the intersection, before swinging down Swanston Street, turning left up Bourke Street and accelerating into Bourke Street Mall.

Faced with crowds of lunchtime shoppers, and tourists taking a break from the Australian Open tennis, the man didn't hesitate.

He bore down, speeding through the mall, and mowing down pedestrians with police in pursuit.

Witness Rebecca Russo told Fairfax Media she saw three people hit.

"We were just standing on Bourke Street, and we heard a bit of a crash coming from the mall," she said.

"I saw a maroon car driving up the pedestrian path on the Flinders Street side. People were running out of the way, they were going very, very fast. I saw him hit a few people. I saw bodies flying into the air."

As he headed west, the man drove in a straight line along the footpath, deliberately ploughing through crowds of people, police said. Men, women and children ran for their lives. He hit dozens.

Kylie Clark, who had taken her 10-year-old daughter Ingrid to the CBD for an Australian Open promotion were standing on the steps of H&M when they saw a maroon car tearing down the footpath on the opposite side of Bourke Street.

"Bodies were flying across the windscreen, and over the back, others were under the car," Ingrid said.

"He was going like a bat out of hell. Didn't care who he collected. Straight up the footpath," Kylie said. "People were screaming. It was chaotic."

As witnesses scattered, running for their lives, others ran towards the chaos, to help however they could.

Sharn Baylis, from Adelaide, said she gave CPR to a young man in his 20s, who appeared to have suffered severe head injuries.

Despite the best efforts of Ms Baylis and others, the man died on the footpath outside Bardot on Bourke Street.

It's believed he had been run over.

"There were massive internal injuries, he was bleeding heavily but the head injury alone probably would have killed him," she said.

According to police, this was no act of terrorism - at least not in the sense we have come to understand it. He wasn't motivated by politics or religion, but by something else.

The 26-year-old man has a history of mental health issues, drug issues, and family violence.

On Friday, he brought his internal chaos and violence to a city, and would leave it changed forever.

- with Tom Cowie, Timna Jacks, Henrietta Cook, Rania Spooner and Neelima Choahan